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Author Topic: Doppler DF with loaded antennas  (Read 6916 times)
W3JKS
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« on: March 06, 2010, 12:18:56 PM »

I have a question for those of you with direction finders based on Doppler.

Is it possible to utilize multi-band (i.e., 150/450/800 MHz) antennas with a wideband receiver in a Doppler setup, assuming that all of the antennas are identical, etc?

73s,
john W3JKS/AAT3BF/AAM3EDE/AAA9SL
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WB6BYU
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« Reply #1 on: March 06, 2010, 09:04:11 PM »

It is - theoretically.  There are a number of potential problems with that approach, however.
First, the spacing has to be closer than is optimum on the lower bands for the array to work
at 800 MHz.  So your 150 MHz bearings won't be as accurate.

Second, the antennas need to have very good phase matching.  This isn't difficult to do
with quarter wave whips, but once you start adding loading coils for multi-band operation
there is little guarantee that the phase will be consistent enough for good results, especially
at 800 MHz, even with nominally identical commercial antennas. Part of this problem comes
from the variation in reactance vs. length:  reactance changes relatively slowly with small
changes in length when an antenna is close to quarter wave resonance, and faster when
it is shorter or longer.  When you add the typical coils and other elements used to make
an antenna work on multiple bands, you've got more uncertainty.

It may be possible to carefully tune all of your antennas to the same electrical performance.
To do this, create a fixture where you can feed both antennas in out of phase.  Choose a
reference antenna and then adjust the second antenna for a null broadside to the two
antennas.  Swap them back and forth to confirm that they are identical, and that the
null in the pattern remains in the same spot.  Then put each of the other antennas into
the fixture in turn and adjust it to match the reference antenna on each band.  Then
you just hope that they all flex in the same direction at the same time when mounted on
the roof.

You also have to make sure that there are no frequency-specific lengths in your antenna
switching circuit.  Some of the variants use quarter wave lines to make the antennas look
like open circuits when they aren't connected.  While you might make this work on 150 and
450 MHz with the same lines, I doubt it will work on 800 MHz at the same time.  Joe Moell
K0OV published a non-frequency-specific design for switched Doppler antennas - you
might look for more information about it on his web site at www.HomingIn.com

I suspect you will find that the antennas are rather closely coupled on 150 MHz when they
are spaced close enough to be used on 800 MHz.  You'll probably have better results with
interchangeable arrays for each band, but you certainly can give it a try.
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W3JKS
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« Reply #2 on: March 07, 2010, 05:52:01 AM »

Thanks!  That website is a great starting point (and I just ordered the book mentioned on the site -- got to love Amazon).

73s,
john W3JKS/AAT3BF/AAM3EDE/AAA9SL
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